The optical properties of the F center in CaF2, which constituted a prominent defect of the material. The F center of CaF2 exhibits a defect state deep in the band-gap. Its excitation by light (at 3.3eV excitation energy) was described by ab initio many-body perturbation theory (GW approximation and the Bethe–Salpeter equation), including electronic exchange, correlation, and electron-hole interaction effects. The excitation causes strong relaxation of the defect geometry, leading to significant broadening of the optical spectrum by 0.5eV and to a large Stokes shift of 1.5eV.

Optical Excitation of Deep Defect Levels in Insulators Within Many-Body Perturbation Theory - the F Center in Calcium Fluoride. Y.Ma, M.Rohlfing: Physical Review B, 2008, 77[11], 115118 (4pp)